Why are a few dioceses smaller in area whilst others are large?

Dioceses are designed with population distribution and size of the country in mind. Special considerations may be certain peoples and cultures that live in the area and which should be treated as a whole. In larger countries, with smaller population centres, such as Canada, the dioceses can be quite big, since one bishop should be able to serve a sparse, though highly distributed population. In densely populated centres, the diocese can be quite small and concentrated, like in Rome. Sometimes, because of a see's particular prestige or important, the area may be incredibly small or large. There are also token sees. For instance, there are bishops who have sees in countries they cannot go and which have no Catholics, such as the Muslim dominated countries of northern African. These are token sees which exist on paper but which cannot be serviced for lack of access. These sees once existed, but were wiped out by regimes or religions not tolerant of Catholicism. The sees remain in the expectation that one day they may be serviced again, not to mention that they are political convenient for jurisdictional purposes in the inner-politics of the Church.